1. Field of the Invention
This invention relates to a method for preventing (meth)acrylic acid and their esters from polymerizing and a method for the production thereof and more particularly to a method for preventing polymerization of (meth)acrylic acid and their esters, characterized by supplying an N-oxyl compound, an N-hydroxy-2,2,6,6-tetramethylpiperidine compound, a 2,2,6,6-tetramethylpiperidine compound, and an N-nitrosophenylhydroxylamine or a salt as a polymerization inhibitor to a distilling column and a method for the production of (meth)acrylic acid or their esters by including the method for the prevention of polymerization in the step of production. This invention also relates to a method for preventing polymerization of (meth)acrylic acid and their esters, characterized by using a nitroso compound as a polymerization inhibitor and performing the distillation in the column while supplying the polymerization inhibitor to the column from the bottom side of the column and a method for the production of (meth)acrylic acid and their esters, including the method for the prevention of the polymerization in the step of production.
2. Description of Related Arts
Such easily polymerizing compounds as acrylic acid and methacrylic acid are raw materials for commercial production and are chemical substances which are produced in a large amount at a plant of a large scale. In the case of (meth)acrylic acid, for example, the easily polymerizing compound is produced by the reaction of catalytic gas phase oxidation of propylene, isobutylene, t-butanol, methyl-t-butyl ether, or acrolein. The reaction gas obtained by the reaction of catalytic gas phase oxidation contains (meth)acrylic acid, the target product, as mixed with other by-products. For example, this reaction generates mainly non-condensable gases, namely unaltered propylene, isobutylene, and acrolein, low-boiling point organic compounds having lower boiling points than acrylic acid, namely steam and unaltered acrolein, formaldehyde produced by a secondary reaction, and impurities such as acetic acid, and high-boiling point compounds having higher boiling points than acrylic acid, namely maleic anhydride, furfural, benzaldehyde, benzoic acid, and acrylic acid dimer. For the purpose of purifying this reaction gas thereby producing the target product, therefore, it is generally for the reaction gas to wash with a counter flow water or a heavy solvent thereby effecting absorption of the gas and subsequently supplying the absorbed gas to a distilling column and purifying it therein.
(Meth)acrylic acid and esters thereof are compounds which possess a very easily polymerizing quality because of their structures. Moreover, since the process for the distillation of (meth)acrylic acid, for example, forms a system in which a gas phase part and a liquid phase part exist in a mixed state, it becomes necessary to prevent both the liquid phase part and the gas phase part in the distilling column effectively from polymerizing and enable the column to be continuously operated stably for a longtime. Generally, for the purpose of preventing the occurrence of such polymerization, various polymerization inhibitors are incorporated in the monomers either singly or in the form of a combination of several members to prevent the process of production from generating a polymer.
In the official gazette of U.S. Pat. No. 5,856,568, for example, discloses a method for preventing a vinyl compound from polymerizing by using an N-nitrosophenylhydroxylamine, i.e. one kinds of nitroso compound, or a salt, characterized by using the N-nitrosophenylhydroxylamine or the salt thereof in the presence of a copper salt compound. This method is claimed to prevent effectively the polymerization of acrylic acid or methacrylic acid and allow the process of production to be operated stably for a long time by introducing the both compounds mentioned above simultaneously or separately into the steps of distillation. In a working example adduced therein, the occurrence of a polymer was observed when a refluxing operation was performed after copper dibutyldithiocarbamate and N-nitrophenyl hydroxyl amine had been dissolved in acrylic acid.
The official gazette of JP-A-08-48,650 (relevant to U.S. Pat. No. 5,504,243) discloses a method for (meth)acrylic acid or a salt thereof from polymerizing by using together with an N-oxyl compound at least one polymerization inhibitor selected from among manganese salt compounds, copper salt compounds, 2,2,6,6-tetramethylpiperidine compounds, and nitroso compounds.
The method described in the official gazette of JP-A-09-95,465 (relevant to U.S. Pat. No. 5,856,568), however, uses a copper salt compound as an essential component and consequently suffers the used water containing the copper compound to be released into the environment. This method, herefore, is at a disadvantage in newly necessitating a means for the disposal of this polluted used water.
Further, for the purpose of effectively preventing (meth)acrylic acid from polymerizing, it suffices to increase the amount of the polymerization inhibitor. This measure, however, entails the disadvantage of necessitating a step for the removal of the polymerization inhibitor at the stage of an operation for polymerizing (meth)acrylic acid, for example.
The official gazette of U.S. Pat. No. 5,504,243 described working examples using a plurality of polymerization inhibitors in combination. The compound stated therein as actually used in combination with an N-nitrosophenylhydroxylamine or a salt thereof is limited to 4-hydroxy-2,2,6,6-tetramethylpiperidinooxyl.
Generally, for the purpose of separating such by-products as low-boiling point substances like acetic acid and some aldehyde and high-boiling point substances like furfural and maleic anhydride which are contained in the (meth)acrylic acid synthesized by the catalytic gas phase oxidation of propylene, for example, the (meth)acrylic acid-containing solution is subjected to distillation, dissipation, extraction, and crystallization. The process production, however, is required to be further simplified. Such in the true status of affairs, when the number of acetic acid separating columns or the number of steps of distillation is decreased by way of simplification, however, the decrease entails the necessity for rigidifying the conditions of distillation. Though the simplification of the process of production constitutes a demand from a social cause, it will be added an occurrence of generation of a polymer because it requires at each step of purification severer conditions of distillation than today. When the polymer is generated, the continuous operation of the purifying column is disabled and the work of removing the polymer from the purifying column is rendered more difficult.
In the light of this true state of affairs, the need for developing in connection with the production of such an easily polymerizing substance as (meth)acrylic acid a method for effectively preventing the occurrence of a polymer in both the gas phase and the liquid phase in a purifying column and consequently attaining continuous operation of the column required inherently and a method for producing the easily polymerizing substance has been earnestly felt.
The present inventors, as a result of pursuing an elaborate study on the function of a nitroso compound manifested in the prevention of polymerization, have found that an N-nitrosophenylhydroxylamine or a salt thereof is decomposed at least partly after it has been added to the step of distillation, the product of this decomposition contains what is possessed of the function of inhibiting polymerization and what is possessed of the function of promoting polymerization in a mixed state, the use of a known N-oxyl compound and other polymerization inhibitor in combination with the decompositions can promote the effect of preventing the polymerization of (meth)acrylic acid, etc., and the supply of the aforementioned compound via a specified point during the introduction thereof into a distilling column can promote the effect of preventing polymerization in the gas phase and repress the effect of the polymerization-promoting substance. This invention has been perfected as a result. Specifically, the object mentioned above is accomplished by the following items (1) to (3).
(1) A method for preventing the polymerization of (meth)acrylic acid and esters thereof, characterized by using an N-nitrosophenylhydroxylamine or a salt thereof in combination with an N-oxyl compound, an N-hydroxy-2,2,6,6-tetramethylpiperidine compound, and a 2,2,6,6-tetramethylpiperidine compound in a purifying column.
(2) The method described above, wherein the N-nitrosophenylhydroxylamine or the salt thereof, mentioned above is supplied to the purifying column from a level not higher than the position of 70% of the total number of theoretical plates, with the bottom side of the column as the base point.
(3) A method for the production of (meth)acrylic acid or an ester thereof, characterized by incorporating in the process of production the method for the prevention of the polymerization of (meth)acrylic acid and esters thereof as set forth in any of Items (1)-(2).
When the N-nitrosophenylhydroxylamine or the salt thereof is used in combination with an N-oxyl compound, an N-hydroxy-2,2,6,6-tetramethylpiperidine compound, and a 2,2,6,6-tetramethylpiperidine compound in accordance with this invention, the N-nitrosophenylhydroxylamine or the salt thereof is decomposed in a distilling column and the gas component consequently obtained effectively prevents the polymerization of the easily polymerizing substance existing in the gas phase of the interior of a purifying column and, at the same time, inhibits the polymerization of the easily polymerizing substance by allowing the presence of the N-oxyl compound in the liquid phase thereof. When the N-nitroso compounds mentioned above are supplied to the purifying column at a level not higher than the position of 70% of the total number of theoretical plates in this case, these compounds are decomposed in the distilling column and the gas component consequently obtained effectively prevents the polymerization of the easily polymerizing substance existing in the gas phase of the interior of a purifying column and, at the same time, inhibits the effect of the aforementioned polymerization promoting substance. Thus, the polymerization of the easily polymerizing substance in both the liquid phase and the gas phase can be synergistically inhibited.
The easily polymerizing compounds contemplated by this invention are (meth)acrylic acid and esters thereof which form both the gas and liquid phases in the interior of the purifying column.
When the (meth)acrylic acid and the esters thereof are produced by implementing the method of this invention for preventing the polymerization, the occurrence of a polymer the process of production is inhibited to permit continuous operation of the column and improve the yield of the product.